When you go to a restaurant you order from a menu, see the price, and pay after you get fed. You don’t send a check the day before your reservation to pay for labor costs in the restaurant kitchen and show up the next day and get served whatever they made (or nothing if the recipe didn’t work out).

Sounds crazy right? That business model just wouldn’t work – we buy food as a good, paying for exactly what we want at a known price – not a service which is the process to get there.